Dual wire welding or additive manufacturing system and method

ABSTRACT

A welding or additive manufacturing system includes a contact tip assembly having first and second exit orifices. A wire feeder is configured to deliver a first and second wire electrodes through the exit orifices. An arc generation power supply is configured to output a current waveform to the wire electrodes simultaneously, through the contact tip assembly. The current waveform includes a bridging current portion, and a background current portion having a lower current level than the bridging current portion. The bridging current portion has a current level sufficient to form a bridge droplet between the wire electrodes before the bridge droplet is transferred to a molten puddle during a deposition operation. Solid portions of the wire electrodes do not contact each other during the deposition operation. The bridge droplet is transferred to the molten puddle during a short circuit event between the molten puddle and the wire electrodes.

BACKGROUND OF THE INVENTION Field of the Invention

Devices, systems, and methods consistent with the invention relate to material deposition with a dual wire configuration.

Description of Related Art

When welding, it is often desirable to increase the width of the weld bead or increase the length of the weld puddle. There can be many different reasons for this desire, which are well known in the welding industry. For example, it may be desirable to elongate the weld puddle to keep the weld and filler metals molten for a longer period of time so as to reduce porosity. That is, if the weld puddle is molten for a longer period of time there is more time for harmful gases to escape the weld bead before the bead solidifies. Further, it may desirable to increase the width of a weld bead so as to cover wider weld gap or to increase a wire deposition rate. In both cases, it is common to use an increased electrode diameter. The increased diameter will result in both an elongated and widened weld puddle, even though it may be only desired to increase the width or the length of the weld puddle, but not both. However, this is not without its disadvantages. Specifically, because a larger electrode is employed more energy is needed in the welding arc to facilitate proper welding. This increase in energy causes an increase in heat input into the weld and will result in the use of more energy in the welding operation, because of the larger diameter of the electrode used. Further, it may create a weld bead profile or cross-section that is not ideal for certain mechanical applications. Rather than increasing the diameter of the electrode, it may be desirable to use at least two smaller electrodes simultaneously.

BRIEF SUMMARY OF THE INVENTION

The following summary presents a simplified summary in order to provide a basic understanding of some aspects of the devices, systems and/or methods discussed herein. This summary is not an extensive overview of the devices, systems and/or methods discussed herein. It is not intended to identify critical elements or to delineate the scope of such devices, systems and/or methods. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later.

In accordance with one aspect of the present invention, provided is a welding or additive manufacturing system. The system includes a contact tip assembly having a first exit orifice and a second exit orifice. A wire feeder is configured to simultaneously deliver a first wire electrode through the first exit orifice of the contact tip assembly and a second wire electrode through the second exit orifice of the contact tip assembly. An arc generation power supply is configured to output a current waveform to the first wire electrode and the second wire electrode simultaneously through the contact tip assembly. The current waveform includes a bridging current portion, and a background current portion having a lower current level than the bridging current portion. The bridging current portion has a current level sufficient to form a bridge droplet between the first wire electrode and the second wire electrode before the bridge droplet is transferred to a molten puddle during a deposition operation. Solid portions of the first wire electrode delivered through the first exit orifice do not contact solid portions of the second wire electrode delivered through the second exit orifice during the deposition operation. The bridge droplet is transferred to the molten puddle during a short circuit event between the molten puddle and the first and second wire electrodes.

In accordance with another aspect of the present invention, provided is a welding or additive manufacturing method. The method includes the step of providing a current waveform to a contact tip assembly having a first exit orifice and a second exit orifice. The current waveform includes a bridging current portion, and a background current portion having a lower current level than the bridging current portion. A first wire electrode is fed through the first exit orifice of the contact tip assembly, and simultaneously a second wire electrode is fed thorough the second exit orifice of the contact tip assembly. During the during the bridging current portion of the current waveform, a bridge droplet is formed coupling the first wire electrode to the second wire electrode before the bridge droplet is transferred to a molten puddle during a deposition operation. Solid portions of the first wire electrode delivered through the first exit orifice do not contact solid portions of the second wire electrode delivered through the second exit orifice during the deposition operation. The bridge droplet is transferred to the molten puddle during a short circuit event between the molten puddle and the first and second wire electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the invention will become apparent to those skilled in the art to which the invention relates upon reading the following description with reference to the accompanying drawings, in which:

FIG. 1 illustrates a welding system;

FIG. 2 illustrates a contact tip assembly;

FIGS. 3A to 3C illustrate diagrammatical representations of a welding operation;

FIGS. 4A to 4B illustrate diagrammatical representations current and magnetic field interactions in a deposition operation;

FIG. 5 is an alternative embodiment of a contact tip assembly;

FIG. 6 shows a portion of a welding torch;

FIG. 7 is a perspective view of a contact tip and diffuser;

FIG. 8 is a perspective view of a contact tip;

FIG. 9 is a perspective view of a contact tip;

FIG. 10 is a perspective view of a diffuser;

FIG. 11 is a perspective view of a diffuser;

FIG. 12 is a perspective view of the example drive roll;

FIG. 13 illustrates a cross section of drives rolls feeding dual wires;

FIG. 14 illustrates a diagrammatical representation of an exemplary welding waveform and wire feeding speeds;

FIG. 15 illustrates a diagrammatical representation of a further exemplary welding waveform and wire feeding speeds; and

FIG. 16 is a flow diagram of an example deposition operation.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the invention will now be described below by reference to the attached Figures. The described exemplary embodiments are intended to assist the understanding of the invention, and are not intended to limit the scope of the invention in any way. Like reference numerals refer to like elements throughout.

As used herein, “at least one”, “one or more”, and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “one or more of A, B, or C” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. Any disjunctive word or phrase presenting two or more alternative terms, whether in the description of embodiments, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” should be understood to include the possibilities of “A” or “B” or “A and B.”

While embodiments of the present invention discussed herein are discussed in the context of GMAW type welding, other embodiments of the invention are not limited thereto. For example, embodiments can be utilized in SAW and FCAW type welding operations, as well as other similar types of welding operations. Further, while the electrodes described herein are solid electrodes, again, embodiments of the present invention are not limited to the use of solid electrodes as cored electrodes (either flux or metal cored) can also be used without departing from the spirit or scope of the present invention. Further, embodiments of the present invention can also be used in manual, semi-automatic and robotic welding operations. Because such systems are well known, they will not be described in detail herein.

Turning now to the Figures, FIG. 1 depicts an exemplary embodiment of a welding system 100 in accordance with an exemplary embodiment of the present invention. The welding system 100 contains an arc generation power supply, such as welding power source 109, which is coupled to a welding torch 111 via a wire feeder 105. The power source 109 can be any known type of welding power source capable of delivering the current and welding waveform, such as pulse spray, STT and/or short arc type welding waveforms. Because the construction, design and operation of such power supplies are well known, they need not be described in detail herein. The power source 109 outputs welding waveforms to wire electrodes E1 and E2 simultaneously, during a dual wire welding operation, via a contact tip in the welding torch 111. The power source 109 can include a controller 120 which is coupled to a user interface to allow a user to input control or welding parameters for the welding operation. The controller 120 can have a processor, CPU, memory etc. to be used to control the operation of the welding process as described herein. The torch 111, which can be constructed similar to known manual, semi-automatic or robotic welding torches can be coupled to any known or used welding gun and can be of a straight or gooseneck type. The wire feeder 105 draws the electrodes E1 and E2 from electrode sources 101 and 103, respectively, which can be of any known type, such as reels, spools, containers or the like. The wire feeder 105 is of a known construction and employs drive rolls 107 to draw the electrodes E1 and E2 and push the electrodes to the torch 111. In an exemplary embodiment of the present invention, the drive rolls 107 and wire feeder 105 are configured for a single electrode operation. Embodiments of the present invention, using a dual wire configuration, can be utilized with a wire feeder 105 and drive rolls 107 only designed for a single wire feeding operation. For example, drive rolls 107 can be configured for a single 0.045 inch diameter electrode, but will suitable drive two electrodes of a 0.030 inch diameter without modification to the wire feeder 105 or the drive rolls 107. Alternatively, the wire feeder 105 can be designed to provide separate sets of rollers for feeding the electrodes E1/E2 respectively, or have drive rolls specifically configured for feeding two or more electrodes simultaneously (e.g., via trapezoidal-shaped wire receiving grooves around the rollers that can accommodate two electrodes). In other embodiments, two separate wire feeders can also be used. As shown, the wire feeder(s) 105 is in communication with the power source 109 consistent with known configurations of welding operations.

Once driven by the drive rolls 107, the electrodes E1 and E2 are passed through a liner 113 to deliver the electrodes E1 and E2 to the torch 111. The liner 113 is appropriately sized to allow for the passage of the electrodes E1 and E2 to the torch 111. For example, for two 0.030 inch diameter electrodes, a standard 0.0625 inch diameter liner 113 (which is typically used for a single 0.0625 inch diameter electrode) can be used with no modification.

Although the examples referenced above discuss the use of two electrodes having a same diameter, the present invention is not limited in this regard as embodiments can use electrodes of a different diameter. That is, embodiments of the present invention can use an electrode of a first, larger, diameter and an electrode of a second, smaller, diameter. In such an embodiment, it is possible to more conveniently weld two work pieces of different thicknesses. For example, the larger electrode can be oriented to the larger work piece while the smaller electrode can be oriented to the smaller work piece. Further, embodiments of the present invention can be used for many different types of welding operations including, but not limited to, metal inert gas, submerged arc, and flux-cored welding. Further, embodiments of the present invention can be used for automatic, robotic and semi-automatic welding operations. Additionally, embodiments of the present invention can be utilized with different electrode types. For example, it is contemplated that a cored electrode can be coupled with a non-cored electrode. Further, electrodes of differing compositions can be used to achieve the desired weld properties and composition of the final weld bead. Thus, embodiments of the present invention can be utilized in a broad spectrum of welding operations.

FIG. 2 depicts an exemplary contact tip assembly 200 of the present invention. The contact tip assembly 200 can be made from known contact tip materials and can be used in any known type of welding gun. As shown in this exemplary embodiment, the contact tip assembly has two separate channels 201 and 203 which run the length of the contact tip assembly 200. During welding a first electrode E1 is passed through the first channel 201 and the second electrode E2 is passed through the second channel 203. The channels 201/203 are typically sized appropriately for the diameter of wire that is to be passed there through. For example, if the electrodes are to have the same diameter the channels will have the same diameters. However, if different diameters are to be used then the channels should be sized appropriately so as to properly transfer current to the electrodes. Additionally, in the embodiment shown, the channels 201/203 are configured such that the electrodes E1/E2 exit the distal end face of the contact tip 200 in a parallel relationship. However, in other exemplary embodiments the channels can be configured such that the electrodes E1/E2 exit the distal end face of the contact tip such that an angle in the range of +/−15° exists between the centerlines of the respective electrodes. The angling can be determined based on the desired performance characteristics of the welding operation. It is further noted that in some exemplary embodiments the contact tip assembly can be a single integrated contact tip with a channels as shown, while in other embodiments the contact tip assembly can be comprised of two contact tip subassemblies located close to each other, where the current is directed to each of the contact tip subassemblies.

As shown in FIG. 2, the respective electrodes E1/E2 are spaced by a distance S which is the distance between the closest edges of the electrodes. In exemplary embodiments of the present invention, this distance is in the range of 0.25 to 4 times the diameter of the larger of the two electrodes E1/E2, while in other exemplary embodiments the distance S is in the range of 2 to 3 times the largest diameter. For example, if each of the electrodes has a diameter of 1 mm, the distance S can be in the range of 2 to 3 mm. In other exemplary embodiments, the distance S is in the range of 0.25 to 2.25 times the diameter of one of the wire electrodes, such as the larger of the two electrodes. In manual or semi-automatic welding operations the distance S can be in the range of 0.25 to 2.25 times the largest electrode diameter, whereas in robotic welding operations the distance S can be in the same or another range, such as 2.5 to 3.5 times the largest electrode diameter. In exemplary embodiments, the distance S is in the range of 0.2 to 3.5 mm.

The wire electrodes E1/E2 project from exit orifices on the end face of the contact tip 200. The diameter of the exit orifices is slightly larger than the diameter of the wire electrodes E1/E2. For example, for a 0.035 inch wire, the diameter of the exit orifice could be 0.043 inches (1.09mm); for a 0.040 inch wire, the diameter of the exit orifice could be 0.046 inches (1.17 mm); for a 0.045 inch wire, the diameter of the exit orifice could be 0.052 inches (1.32 mm). The channels 201, 203 and exit orifices are spaced appropriately to facilitate the formation of a single bridge droplet between the wire electrodes E1/E2 during a deposition operation. For exit orifices sized for electrodes having a diameter 0.045 inches and smaller, the distance between the exit orifices (inner circumference to inner circumference, similar to distance S) can be less than 3 mm to facilitate the formation of a bridge droplet. However, spacing of 3 mm or greater between the exit orifices may be possible, depending on the wire size, magnetic forces, orientation (e.g., angle) of the channels 201, 203, etc. In certain embodiments, the distance between the exit orifices is within the range of 20% to 200% of the diameter of one or both of the exit orifices, which can also correspond to the distance S between the wire electrodes being in the range of 0.25 to 2.25 times the diameter of the electrodes.

As explained further below, the distance S should be selected to ensure that a single bridge droplet is formed between the electrodes E1/E2, before the droplet is transferred to the molten puddle during a deposition operation, while also preventing the solid portions of the electrodes E1/E2 that are delivered through the exit orifices from contacting each other, other than through the bridge droplet.

FIG. 3A depicts an exemplary embodiment of the present invention, while showing the interactions of the magnetic forces from the respective electrodes E1 and E2. As shown, due to the flow of current, a magnetic field is generated around the electrodes which tends to create a pinch force that draws the wires towards each other. This magnetic force tends to create a droplet bridge between the two electrodes, which will be discussed in more detail below.

FIG. 3B shows the bridge droplet that is created between the two electrodes E1/E2. That is, as the current passing through each of the electrodes melts the ends of the electrodes the magnetic forces tend to draw the molten droplets at the ends of the electrodes E1/E2 towards each other until they connect with each other. The distance S is far enough such that the solid portions of the electrodes are not drawn to contact each other during the deposition operation, but close enough that a bridge droplet is created between the electrodes, before the molten droplet is transferred to the weld puddle created by the welding arc. It can be seen in FIG. 3B that a large bridge droplet can form between the electrodes E1/E2; however, there is a relatively small cross sectional area between the droplet and the electrodes. That is, the electrodes contact the bridge droplet along a small cross sectional area of the droplet. In certain embodiments, the bridge droplet is transferred to the molten puddle during a short circuit event between the molten puddle and the wire electrodes E1/E2. This process is known as short arc welding. The short circuit event is depicted in FIG. 3C. The electrodes E1/E2 are driven by the wire feeder toward the workpiece W at a wire feed speed (WFS) that is sufficient to ensure that the bridge droplet does not detach (e.g., pinch off from the electrodes) before the droplet and electrodes short to the molten puddle on the workpiece W. When the force of feeding the electrodes E1/E2 overcomes the heating of the arc and the bridge droplet touches the molten puddle as shown, the large bridge droplet is drawn into the weld pool, and surface tension along with pinch force transfers the droplet from the electrodes to the weld pool. The size of the bridge droplet is significantly larger than the droplets formed in conventional, single electrode short arc welding. This results in a deposit rate per short arc cycle that exceeds conventional short arc welding, yet the cross sectional area between the electrodes and the droplet is relatively small, requiring less pinch force to transfer that droplet. After the bridge droplet shorts to the molten puddle, the electrodes E1/E2 can continue to be driven into the molten puddle for a brief time while being resistive heated by the welding current. Sufficient heat will be present in the molten puddle, in combination with the resistive heating, to melt the electrodes E1/E2 and allow them to be consumed into the weld pool as additional filler metal.

FIG. 4A depicts an exemplary representation of current flow in an embodiment of the present invention. As shown, the welding current is divided so as to flow through each of the respective electrodes and passes to and through the bridge droplet as it is formed. The current then passes from the bridge droplet to the puddle and work piece. In exemplary embodiments where the electrodes are of the same diameter and type the current will be essentially divided evenly through the electrodes. In embodiments where the electrodes have different resistance values, for example due to different diameters and/or compositions/construction, the respective currents will be apportioned due to the relationship of V=I*R, as the welding current is applied to the contact tip similar to known methodologies and the contact tip provides the welding current to the respective electrodes via the contact between the electrodes and the channels of the contact tip. FIG. 4B depicts the magnetic forces that aid in creating the bridge droplet. As shown, the magnetic forces tend to pull the respective molten portions of the electrodes towards each other until they contact with each other.

FIG. 5 depicts an alternative exemplary embodiment of a contact tip 700 that can be used with embodiments of the present invention. As described previously, in some embodiments the electrodes can be directed to the torch via a single wire guide/liner. Of course, in other embodiments, separate wire guide/liners can be used. However, in embodiments in which a single wire guide/liner is used, the contact tip can be designed such that the electrodes are separated from each other within the contact tip. As shown in FIG. 5, this exemplary contact tip 700 has a single entrance channel 710 with a single orifice at the upstream end of the contact tip 700. Each of the electrodes enter the contact tip via this orifice and pass along the channel 710 until they reach a separation portion 720 of the contact tip, where the separation portion directs one electrode into a first exit channel 711 and a second electrode into the second exit channel 712, so that the electrodes are directed to their discrete exit orifices 701 and 702, respectively. Of course, the channels 710, 711 and 712 should be sized appropriately for the size of electrodes to be used, and the separation portion 720 should be shaped so as to not scar or scratch the electrodes. As shown in FIG. 5, the exit channels 711 and 712 are angled relative to each other, however, as shown in FIG. 2, these channels can also be oriented parallel to each other.

As discussed above, the wire electrodes used in a multi-wire deposition operation (e.g., welding, additive manufacturing, hardfacing, etc.) can be spaced by a distance S that facilitates formation of a bridge droplet between the wire electrodes. The size of the bridge droplet is determined by the spacing between the wire electrodes and the spacing between the exit orifices in the contact tip. The size of the bridge droplet determines the width of the electric arc that exists during the deposition operation, and reducing the spacing between the exit orifices and wire electrodes narrows the arc width. Larger bridge droplets may be preferred for larger welds, and smaller bridge droplets preferred for smaller welds. Deposition rate is impacted by the arc width, and the deposition rate for small gauge wires can be increased by reducing the spacing between the exit orifices and wire electrodes (e.g., from approximately 2 mm to 1 mm).

The maximum spacing between the exit orifices and between the wire electrodes is reached when the magnetic forces developed by the current waveform (e.g., at the peak current level) still allow formation of the bridge droplet, and is exceeded when bridging is no longer possible. The minimum spacing is that which keeps the solid portions of the wires separated at the point of bridging. The magnetic forces tend to pull the wire electrodes together, and the wires are somewhat flexible. Thus, the minimum spacing between the exit orifices and between the wire electrodes will depend on the stiffness of the electrodes, which is impacted by parameters such as wire diameter, material of construction, etc.

FIG. 6 depicts an end portion of an exemplary welding torch in accordance with the present invention. Because the construction and operation of welding torches is generally known, the details of such construction and operation will not be discussed in detail herein. As shown, the torch includes a number of components and is used to deliver at least two wire electrodes and a shielding gas to a workpiece for a welding or additive manufacturing operation. The torch includes a diffuser 205 which aids in properly directing and distributing the shielding gas for a welding operation. Coupled to the downstream end of the diffuser 205 is a contact tip 200, which is used to pass the welding current into the at least two wire electrodes which are passing though the contact tip simultaneously during welding. The contact tip 200 is configured to facilitate the formation of a bridge droplet between the wire electrodes that are delivered through bores or channels in the contact tip. The bridge droplet couples the first wire electrode to the second wire electrode prior to contacting a molten puddle created by the deposition operation, as discussed above.

Threaded onto the outside of the diffuser 205 is an insulator 206. The insulator 206 electrically isolates a nozzle 204 from the electrically live components within the torch. The nozzle 204 directs the shielding gas from the diffuser 205 to the distal end of the torch and the workpiece during welding.

Conventional contact tips have threads on an upstream or proximal end of the contact tip that thread into the diffuser. The contact tip and diffuser are connected by screwing the contact tip into the diffuser. Such a fastening system works well for welding with single wires. The welding wire can be threaded through the contact tip and the contact tip can be rotated around the wire multiple times and screwed into the diffuser. However, when welding with multiple welding wires simultaneously passing through the contact tip, such a fastening system would result in an undesirable twisting of the welding wires. For example, if two welding wires are passed through the contact tip, subsequently threading the contact tip onto the diffuser by multiple turns requiring greater than 360° of rotation will result in the welding wires becoming twisted and unable to be fed through the contact tip.

The contact tip 200 in FIG. 6 is attached to the diffuser 205 by rotation of the contact tip through less than 360°, such as 270° (three-quarter turn), 180° (one-half turn), 90° (quarter turn), less than 90°, etc. The rotation of the contact tip 200 necessary to attach the contact tip to the diffuser 205 can be any angle as desired that is preferably less than 360° and results in the multiple wire electrodes passing through the contact tip not becoming unduly twisted during installation of the contact tip. If the welding wires are unduly twisted during installation of the contact tip, wire feeding problems will result and “bird nesting” of the welding wires can occur.

With reference to FIGS. 6-11, the contact tip 200 is attached to the diffuser 205 by a quarter turn, clockwise rotation of the contact tip within the diffuser. The contact tip 200 has a forward or downstream distal portion that has a tapered shape and includes flats 215 to accommodate gripping by a tool, such as pliers. The contact tip 200 has a rearward or upstream proximal portion 208 that is generally cylindrical, but includes a radially-projecting tab 210 that engages a slot 212 in an interior wall of the diffuser 205, to securely connect the contact tip to the diffuser. The rearward portion 208 of the contact tip 200 is located within the diffuser 205 when the contact tip is installed onto the diffuser, and acts as a mounting shank for the contact tip. It can be seen that the diameter of the rearward portion 208 of the contact tip is smaller than the adjacent downstream portion, which results in a shoulder 211 projecting radially from the cylindrical rearward portion 208 of the contact tip. The shoulder 211 seats against the terminal end face of the diffuser 205 when the contact tip 200 is installed onto the diffuser.

The contact tip 200 can be made from known contact tip materials and can be used in any known type of welding gun. The contact tip can comprise an electrically-conductive body, such as copper, extending from its rearward, proximal end to its forward, distal end. As shown in this exemplary embodiment, the contact tip 200 has two separate wire channels or bores 214 and 216 which run the length of the contact tip. The channels 214/216 can extend between wire entrance orifices on the proximal end face of the mounting shank 208, and wire exit orifices on the distal end face of the contact tip. During welding, a first wire electrode is delivered through the first channel 214 and a second wire electrode is delivered through the second channel 216. The channels 214/216 are typically sized appropriately for the diameter of wire that is to be fed through the channel. For example, if the electrodes are to have the same diameter, then the channels will have the same diameters. However, if different wire sizes are to be used together, then the channels should be sized appropriately so as to properly transfer current to the differently-sized electrodes. Additionally, in the embodiment shown, the channels 214/216 are configured such that the electrodes exit the distal end face of the contact tip 200 in a parallel relationship. However, in other exemplary embodiments the channels can be configured so that the electrodes exit the distal end face of the contact tip such that an angle in the range of +/−15° exists between the centerlines of the respective electrodes. The angling can be determined based on the desired performance characteristics of the welding operation. The example contact tips discussed herein are shown having two electrode bores. However, it is to be appreciated that the contact tips could have bores for more than two electrodes, such as three or more bores.

The slot 212 in the interior wall of the diffuser 205 includes an axial portion 218 and a helical portion 220. The axial portion 218 of the slot 212 extends to the downstream terminal end face of the diffuser 205, against which the shoulder 211 of the contact tip 200 seats. After the welding electrodes are fed through the contact tip 200, the radially-projecting tab 210 on the mounting shank 208 is inserted into the axial portion 218 of the 212 slot and the contact tip is pushed into the diffuser 205. When the tab 210 reaches the helical portion 220 of the slot, the contact tip 200 is rotated to move the tab to the end of the helical portion. The helical portion 220 has a slight upstream pitch that draws the contact tip 200 inward as the contact tip is rotated, so that the shoulder 211 of the contact tip seats against the downstream terminal end face of the diffuser 205. The tab 210 on the mounting shank 208 can have a tapered edge 217 that matches the pitch of the slot 212 in the diffuser 205, to help ensure a tight connection between the two components. In the example embodiment shown, the helical portion 220 of the slot 212 allows for a quarter turn of the contact tip 200 to secure the contact tip to the diffuser 205. However, it is to be appreciated that other rotational angles are possible (e.g., more or less than a quarter turn or90°). For example, the helical portion 220 of the slot can extend less than 360° around the inner circumference of the interior chamber of the diffuser 205.

FIG. 12 illustrates an example drive roll 107 for use in a wire feeder in a dual wire welding system. The drive roll 107 has a central bore. The inner surface of the bore can include contoured recesses 131 for receiving projections on a driving mechanism, such as a drive gear, to transfer drive torque to the drive roll 107. The drive roll 107 includes one or more annular or circumferential wire receiving grooves 133, 135. The wire receiving grooves 133, 135 are spaced axially along the circumference of the drive roll 107. The wire receiving grooves 133, 135 are designed to receive two welding wires. Example standard welding wire diameters for use with the drive rolls 107 include 0.030 inches, 0.035 inches, 0.040 inches, 0.045 inches, etc. The wire receiving grooves 133, 135 can have the same width and depth as each other, or have different widths and depths to accommodate different sizes or combinations of dual welding wires. If the wire receiving grooves 133, 135 each have the same width and depth, then the drive roll 107 can be reused when one groove is worn out by simply flipping the drive roll over and reinstalling it on the wire feeder. The wire receiving grooves 133, 135 can be configured to simultaneously drive two wires having the same diameter, or two wires having different diameters. The wire receiving grooves 133, 135 can have a generally trapezoidal shape with straight, angled or inwardly-tapered sidewalls and a flat base extending between the sidewalls. However, the wire receiving grooves 133, 135 could have other shapes besides a trapezoidal shape, such as having a curved, concave groove base for example. In certain embodiments, the grooves 133, 135 can include knurling or other frictional surface treatments to help grip the welding wires.

FIG. 13 shows a partial cross section of two drive rolls 107 as they would be mounted on a wire feeder for supplying dual welding wires during a deposition operation. The drive rolls 107 are biased together to provide a clamping force on the first E1 and the second E2 welding wires. The welding wires E1, E2 are both located in the annular grooves of the upper and lower drive rolls 107. Due to the bias force applied to the drive rolls 107, the welding wires E1, E2 are clamped in the annular grooves between upper and lower sidewalls 150 forming the grooves and the neighboring welding wire. The welding wires E1, E2 are stably held via three points of contact within the annular grooves. This clamping system can allow both wires to be fed through the wire feeder in a consistent manner. The two welding wires E1, E2 support each other during feeding and pull each other along via friction. Because the sidewalls 150 of the annular grooves are angled, they apply both vertical and horizontal clamping forces on the welding wires E1, E2. The horizontal clamping force pushes the welding wires E1, E2 together, causing them to contact each other. The welding wires E1, E2 are clamped within the annular grooves so as to be radially offset from both concave groove bases 152. That is, the welding wires E1, E2 are pinned between each other and the angled sidewalls 150 of the grooves such that gaps exist between each of the welding wires and the groove bases 152. In an example embodiment, the angle between the sidewalls 150 and the outer circumference of the drive roll 107 is about 150°, although other angles are possible and can be determined with sound engineering judgment.

The wire clamping provided by the drive rolls 107 allows for some variability (e.g., due to manufacturing tolerances) in the diameters of the welding wires E1, E2. If each welding wire E1, E2 had its own dedicated annular groove in the drive rolls 107, and one of the welding wires was slightly larger than the other, then the smaller welding wire might not be adequately clamped between the drive rolls. In such a situation, the larger welding wire would limit the radial displacement of the drive rolls 107 toward each other, thereby preventing proper clamping of the smaller wire. This could lead to feeding problems and so-called birdnesting of the smaller welding wire during feeding. The clamping system discussed above can accommodate wires of different sizes because the clamping system is self-adjusting. When one welding wire E1 is larger than the other E2, the contact point between the wires is shifted axially from a central position within the annular grooves toward the smaller wire. Three points of contact are maintained on each welding wire E1, E2 by the sidewalls 150 of the groove and the neighboring welding wire.

FIGS. 14 and 15 provide example current 300 and voltage 320 waveforms for a short arc welding operation using a bridge droplet as described above. FIGS. 14 and 15 also show example changes to wire feed speed (WFS) 340, 360 during the short arc welding operation, and movement of the wire electrodes and the bridge droplet during the welding operation. The periodic short circuit event, during which the bridge droplet is transferred to the molten weld puddle and the wire electrodes are shorted to the workpiece, is indicated in FIGS. 14 and 15 as the interval between “short” and “clear”. After the short circuit event is cleared, a new bridge droplet is formed between the wire electrodes while the electrodes are separated from the weld puddle and workpiece. The electrodes and newly-developed bridge droplet are driven toward the weld puddle by the wire feeder, to transfer another bridge droplet to the weld puddle via another short circuit event. This cycle repeats for the duration of the short arc deposition operation.

The current waveform 300 includes a bridging current portion 302. The bridging current portion 302 is a high current portion of the waveform and is sufficient to form the bridge droplet between the wire electrodes. An example current range for the bridging current portion 302 is 200A-1000A. After the bridging current portion 302, the current can be lowered to a background current portion 304. Example current levels for the background current portion are less than 100A, less than 50A, and about 20A. The lower background current portion 304 reduces the magnetic forces around the bridge droplet and helps to keep the bridge droplet from detaching from the electrodes before a short circuit event occurs (i.e, before the bridge droplet shorts to the molten puddle). During the bridging current portion 302 and the background current portion 304, the WFS is positive (i.e., the wire electrodes are driven toward the molten puddle), as shown. The wire feed speed can be kept constant during the bridging current portion 302 and the background current portion 304, or the WFS be adjusted during these portions of the welding waveform. For example, the WFS can be higher during the bridging current portion 302 and then reduced as the current waveform 300 transitions to the background current 304. The WFS can also be decreasing (e.g., have a negative slope) as the bridge droplet shorts to the molten puddle for a soft or gentle contact when the short occurs. Regardless of whether the WFS is kept constant or altered while the electrodes and bridge droplet are driven toward the molten puddle, the WFS should be high enough to ensure that the bridge droplet is quickly pushed into the molten puddle before it separates from the electrodes.

The current waveform 300 can include a short circuit wire heating portion 306. The short circuit wire heating portion 306 can have a higher current level than the background current portion 304 and a lower current level than the bridging current portion 302. An example current level for the short circuit wire heating portion 304 is about 100A, although higher or lower current levels are possible. For example, the short circuit wire heating portion 304 could be about the same level as the background current portion 304. The short circuit wire heating portion 306 occurs during the short circuit event, after the bridge droplet shorts into the molten puddle. The short circuit wire heating portion 306 serves to resistive heat the electrodes, which are also shorted to the molten puddle, while the arc is extinguished. The resistive heating combined with the heat present in the molten puddle allows the electrodes to be consumed as filler wire in the molten puddle as they are driven into the puddle by the wire feeder. The total heat input of the short circuit wire heating portion 306 is low due to the low voltage of the short circuit. The WFS can remain positive or forward during the majority of the short circuit wire heating portion 306, but could be slowed, stopped or even reversed during the short circuit wire heating portion if desired. The short circuit wire heating portion 306 of the current waveform 300 can be applied for a predetermined duration during the short circuit event, and then the current level can be reduced to a low level 308 before the short clears, to minimize spatter. Rather than applying the short circuit wire heating portion 306 for a predetermined duration, this intermediate current level could be maintained until it is apparent that the short is about to clear, as determined by the welding power source or wire feeder, at which time the current level can be reduced to the low level 308. The short circuit wire heating portion 306 can also be regulated as a heating power level that is maintained for a duration that is sufficient to supply a desired amount of energy (e.g., joules) through the electrodes. The power level can be integrated to determine the total amount of energy supplied during the short circuit wire heating portion 308. When the desired amount of heating energy has been supplied, the current level can be reduced to the low level 308. After the welding power source or wire feeder determines that the short has cleared, or after the low current level 308 has been applied for a duration during which the short is expected to clear, the power source switches from the spatter minimizing low current level 308 to the bridge droplet forming bridging current level 302.

The welding power source and/or wire feeder can detect the occurrence of the short circuit event by monitoring the voltage of the deposition operation (e.g., the welding voltage). In certain embodiments, the welding power source can reduce the welding current upon the occurrence of the short circuit event, to reduce spatter. Such a current reduction is shown by dashed lines on the current trace of FIG. 14. The welding power source and/or wire feeder can also determine when the short is about to clear, such as by monitoring the rate of change of the welding voltage (dV/dt) for example. Based on the rate of change of the welding voltage (dV/dt), WFS can be adjusted and the bridging current level 302 output to the electrodes, in response to the short being clearing.

During arcing portions of the deposition operation, the WFS is positive and relative high to push the bridge droplet into the molten puddle before it detaches from the electrodes. However, during the short circuit event, the WFS can be reduced to allow a pinch force from the current flow to separate the electrodes from the molten puddle and reestablish arcs from the electrodes. In certain embodiments, wire feeding can be stopped during the short circuit event, as shown schematically in FIG. 15 with the WFS reaching zero during the short circuit event. The feeding direction of the wire feeder can also be reversed during the short circuit event (FIG. 14), to pull the electrodes out of the molten puddle. In that case, the wire feeder initially drives the electrodes into the molten puddle during the short circuit event to add additional filler metal to the puddle, and then subsequently pulls the electrodes away from the molten puddle to help reestablish the arcs. The timing for slowing, stopping, and/or reversing the electrodes can be based on the detected occurrence of the short circuit event. For example, the power source can monitor the welding voltage and control the wire feeder to slow/stop/reverse the electrodes when the welding voltage drops to a short circuit level, or slow/stop/reverse the electrodes at a predetermined duration of time after the short circuit occurs. The welding arcs between the electrodes and the molten puddle can be reestablished while the wire feeding is slowed or stopped or the feeding direction is reversed (away from the molten puddle), as shown at the bottom of FIGS. 14 and 15. The wire feeder can then resume delivering the electrodes toward the molten puddle after the short clears and the arcs are reestablished as shown. Changes to the WFS and direction can be coordinated with changes to the current level during the short circuit and arcing portions of the deposition operation.

The frequency of the welding waveforms discussed above can be lower than conventional welding waveforms while maintaining high deposition rates. A conventional welding waveform may have a frequency of about 100 Hz, for example, whereas the waveforms of the present invention may be less than 100 Hz, such between 20 Hz and 60 Hz.

FIG. 16 depicts a flow diagram of an example deposition operation, which may be welding, additive manufacturing, etc. A current waveform is provided to a contact tip assembly having a first exit orifice and a second exit orifice 400. The current waveform includes at least a bridging current portion and a background current portion having a lower current level than the bridging current portion. A first wire electrode is fed through the first exit orifice of the contact tip assembly and simultaneously a second wire electrode is fed thorough the second exit orifice of the contact tip assembly 402. The electrodes are fed by one or more wire feeders, and the feeding speeds can be varied. During the bridging current portion of the current waveform, a bridge droplet is formed before the bridge droplet is transferred to a molten puddle during a deposition operation 404. The bridge droplet couples the first wire electrode to the second wire electrode, and solid portions of the first wire electrode delivered through the first exit orifice do not contact solid portions of the second wire electrode delivered through the second exit orifice. The bridge droplet is transferred to the molten puddle during a short circuit event between the molten puddle and the first and second wire electrodes 406. A feed speed of the first and second wire electrodes is reduced and/or the feeding direction reversed during the short circuit event 408. In certain embodiments, the first and second wire electrodes are initially driven into the molten puddle and subsequently pulled away from the molten puddle during the short circuit event. Feeding of the first and second wire electrodes can also be stopped during the short circuit event, and resumed when the short is cleared. A rate of change of voltage (dV/dt) of the deposition operation can be monitored to determine clearance of the short circuit event from the rate of change of voltage (dV/dt) 410. Arcs are reestablished between the molten puddle and the first and second wire electrodes 412. The arcs can be reestablished while feeding of the electrodes is slowed or stopped, or while the feeding direction of the electrodes is reversed (i.e., away from the molten puddle).

The use of embodiments described herein can provide significant improvements in stability, weld structure and performance over known welding operations. However, in addition to welding operations, embodiments can be used in additive manufacturing operations. In fact the system 100 described above can be used in additive manufacturing operations as well as welding operations. In exemplary embodiments, improved deposition rates can be achieved in additive manufacturing operations using the dual wire short arc techniques described above. Because additive manufacturing processes and systems are known, the details of such processes and systems need not be described herein.

It is noted that exemplary embodiments are not limited to the usage of the waveforms discussed above and described herein, as other welding type waveforms can be used with embodiments of the present invention. For example, other embodiments can use variable polarity or AC waveforms, etc. without departing from the spirit and scope of the present invention. For example, in variable polarity embodiments the bridging current portion of the welding waveform can be done in a negative polarity such that the bridge droplet is created while reducing the overall heat input into the weld puddle.

Embodiments of the present invention can be used with different types and combinations of consumables including flux cored consumables. In fact, embodiments of the present invention can provide a more stable welding operation when using flux cored electrodes. Specifically, the use of a bridging droplet can aid in stabilizing flux core droplets that can tend to be unstable in a single wire welding operation. Further, embodiments of the present invention allow for increased weld and arc stability at higher deposition rates, such as those exceeding 10 lb/hr.

Additionally, as indicated above the consumables can be of different types and/or compositions, which can optimize a given welding operation. That is, the use of two different, but compatible, consumables can be combined to create a desired weld joint. For example, compatible consumables include hardfacing wires, stainless wires, nickel alloys and steel wires of different composition can be combined. As one specific example a mild steel wire can be combined with an overalloyed wire to make a 309 stainless steel composition. This can be advantageous when a single consumable of the type desired does not have desirable weld properties. For example, some consumables for specialized welding provide the desired weld chemistry but are extremely difficult to use and have difficulty providing a satisfactory weld. However, embodiments of the present invention allow for the use of two consumables that are easier to weld with to be combined to create the desired weld chemistry. Embodiments of the present invention can be used to create an alloy/deposit chemistry that is not otherwise commercially available, or otherwise very expensive to manufacture. Thus, two different consumables can be used to obviate the need for an expensive or unavailable consumable. Further, embodiments can be used to create a diluted alloy. For example, a first welding wire could be a common inexpensive alloy and a second welding wire could be a specialty wire. The desired deposit would be the average of the two wires, mixed well in the formation of the bridged droplet, at the lower average cost of the two wires, over an expensive specialty wire. Further, in some applications, the desired deposit could be unavailable due to the lack of appropriate consumable chemistry, but could be reached by mixing two standard alloy wires, mixed within the bridged droplet and deposited as a single droplet. Further, in some applications, such as the application of wear resistance metals, the desired deposit may be combination of tungsten carbide particles from one wire and chrome carbide particles from another. Still in another application, a larger wire housing larger particles within is mixed with a smaller wire containing fewer particles or smaller particles, to deposit a mixture of the two wires. Here the expected contribution from each of the wires is proportional to the size of wire given the wire feed speeds are same. In yet another example, the wire feed speeds of the wires are different to allow the alloy produced to change based on the desired deposit but the mixing of the wires is still produced by the bridge droplet created between the wires.

It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited. 

What is claimed is:
 1. A welding or additive manufacturing system, comprising: a contact tip assembly having a first exit orifice and a second exit orifice; a wire feeder configured to simultaneously deliver a first wire electrode through the first exit orifice of the contact tip assembly and a second wire electrode through the second exit orifice of the contact tip assembly; and an arc generation power supply configured to output a current waveform to the first wire electrode and the second wire electrode simultaneously through the contact tip assembly, wherein: the current waveform includes a bridging current portion, and a background current portion having a lower current level than the bridging current portion, the bridging current portion has a current level sufficient to form a bridge droplet between the first wire electrode and the second wire electrode before the bridge droplet is transferred to a molten puddle during a deposition operation, wherein solid portions of the first wire electrode delivered through the first exit orifice do not contact solid portions of the second wire electrode delivered through the second exit orifice during the deposition operation, and the bridge droplet is transferred to the molten puddle during a short circuit event between the molten puddle and the first and second wire electrodes.
 2. The welding or additive manufacturing system of claim 1, wherein the current waveform includes a short circuit wire heating portion that occurs during the short circuit event, wherein the short circuit wire heating portion has a higher current level than the background current portion and a lower current level than the bridging current portion.
 3. The welding or additive manufacturing system of claim 1, wherein, during the short circuit event, a feeding direction of the wire feeder is reversed such that the first wire electrode and the second wire electrode are initially driven into the molten puddle and subsequently pulled away from the molten puddle by the wire feeder during the short circuit event.
 4. The welding or additive manufacturing system of claim 3, wherein arcs are reestablished between the molten puddle and the first and second wire electrodes while the feeding direction is away from the molten puddle.
 5. The welding or additive manufacturing system of claim 1, wherein, during the short circuit event, a wire feed speed of the wire feeder is reduced.
 6. The welding or additive manufacturing system of claim 5, wherein the wire feeder stops feeding the first wire electrode and the second wire electrode toward the molten puddle during the short circuit event, and resumes feeding the first wire electrode and the second wire electrode toward the molten puddle when the short circuit event is cleared.
 7. The welding or additive manufacturing system of claim 5, wherein one or both of the wire feeder and the arc generation power supply is configured to monitor a rate of change of voltage (dV/dt) of the deposition operation and determine clearance of the short circuit event from said rate of change of voltage (dV/dt).
 8. The welding or additive manufacturing system of claim 7, wherein the arc generation power supply is configured to output the bridging current portion of the welding waveform based on the clearance of the short circuit event.
 9. The welding or additive manufacturing system of claim 1, wherein the wire feeder delivers the first wire electrode and the second wire electrode toward the molten puddle at a first wire feed speed during the bridging current portion of the current waveform, and delivers the first wire electrode and the second wire electrode toward the molten puddle at a second wire feed speed during the background current portion of the current waveform, wherein the second wire feed speed is less than the first wire feed speed.
 10. The welding or additive manufacturing system of claim 1, wherein the first and second exit orifices are separated from each other by a distance configured to facilitate formation of the bridge droplet between the first wire electrode and the second wire electrode.
 11. A welding or additive manufacturing method, comprising the steps of: providing a current waveform to a contact tip assembly having a first exit orifice and a second exit orifice, wherein the current waveform includes a bridging current portion, and a background current portion having a lower current level than the bridging current portion; feeding a first wire electrode through the first exit orifice of the contact tip assembly and simultaneously feeding a second wire electrode thorough the second exit orifice of the contact tip assembly; forming, during the bridging current portion of the current waveform, a bridge droplet coupling the first wire electrode to the second wire electrode before the bridge droplet is transferred to a molten puddle during a deposition operation, wherein solid portions of the first wire electrode delivered through the first exit orifice do not contact solid portions of the second wire electrode delivered through the second exit orifice during the deposition operation; and transferring the bridge droplet to the molten puddle during a short circuit event between the molten puddle and the first and second wire electrodes.
 12. The welding or additive manufacturing method of claim 11, wherein the current waveform includes a short circuit wire heating portion that occurs during the short circuit event, wherein the short circuit wire heating portion has a higher current level than the background current portion and a lower current level than the bridging current portion.
 13. The welding or additive manufacturing method of claim 11, further comprising the step of reversing a feeding direction of the first and second wire electrodes during the short circuit event such that the first and second wire electrodes are initially driven into the molten puddle and subsequently pulled away from the molten puddle during the short circuit event.
 14. The welding or additive manufacturing method of claim 13, further comprising the step of reestablishing arcs between the molten puddle and the first and second wire electrodes while the feeding direction of the first and second wire electrodes is away from the molten puddle.
 15. The welding or additive manufacturing method of claim 11, further comprising the step of reducing a wire feed speed of the first and second wire electrodes during the short circuit event.
 16. The welding or additive manufacturing method of claim 15, further comprising the steps of stopping the feeding of the first and second wire electrodes during the short circuit event, and resuming the feeding of the first and second wire electrodes when the short circuit event is cleared.
 17. The welding or additive manufacturing method of claim 15, further comprising the step of monitoring a rate of change of voltage (dV/dt) of the deposition operation and determining clearance of the short circuit event from said rate of change of voltage (dV/dt).
 18. The welding or additive manufacturing method of claim 17, wherein a timing of the bridging current portion of the welding waveform is based on determining the clearance of the short circuit event.
 19. The welding or additive manufacturing method of claim 11, wherein the step of feeding includes feeding the first wire electrode and the second wire electrode toward the molten puddle at a first wire feed speed during the bridging current portion of the current waveform, and feeding the first wire electrode and the second wire electrode toward the molten puddle at a second wire feed speed during the background current portion of the current waveform, wherein the second wire feed speed is less than the first wire feed speed.
 20. The welding or additive manufacturing method of claim 11, wherein the first and second exit orifices are separated from each other by a distance configured to facilitate formation of the bridge droplet between the first wire electrode and the second wire electrode. 